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I have been asked to review a couple of new Li-ion battery chargers by Xtar. Although I don’t have the experience or expertise of others here, I’ve agreed to compare them to the 3 chargers I keep on hand for regular use: the standard Pila charger, the classic DSD charger, and the basic-model Cytac/HXY/DealExtreme.6105 charger.

The WP2 comes with both standard AC power adapter and DC car adapter. The cabling and overall build seem reasonably good (good length on the cables, reasonably solid, etc.). The battery trays are spring-mounted, and can easily accommodate any size from 14500-18650.

The MP1 is a much simpler package, consisting of the charging base and a short standard micro-USB/USB charging cable. Also included with my review sample were a USB-to-AC adapter and a USB-to-DC adapter.

Note that the bundled USB cable length is rather short. But since it uses the standard micro-USB connector at the charging cradle end, you should be able to use a longer one from one of your other devices (e.g. camera, blackberry, etc.).

I note that the specifications for the two chargers seem similar – but the MP1 also specifically notes that the charger will monitor the cell once fully charged, and maintain a low 4uA charging current. I have heard similar claims made for the WP2, but don’t see that on their spec sheet.

For the charging comparisons, here are the 3 chargers I routinely use:

Pila charger:

The Pila charger is probably the “standard” by which all Li-ion chargers are judged. Well designed, it uses a proper Constant-Current/Constant-Voltage charging regimen with proper termination (i.e. current shuts off once the cell reaches full charge). Features two independent channels. You can also top-up your nearly-full cells by manually forcing a charge (i.e. most chargers won’t charge cells that exceed ~4.0V).

With the bundled spacers, you can charge everything from 14500 and up. Although the Pila will also charge 16340 (RCR) with additional spacers, this really isn’t the ideal charger for them (i.e. the charge and cut-off current may be a bit high for them). However, the Pila should be reasonably safe on RCR thanks to its “intelligent” charging algorithm.

DSD charger:

Despite appearances, this is actually a single-channel charger (i.e. it is wired for 2 cells in parallel, for simultaneous charging). Simple and basic, it is fairly popular for its Constant Current charging method, followed by true termination (i.e. no trickle charge). Main drawback is its slow rated charge rate (~350mA), and no CV stage.

DX.6105 charger:

I often use the common DX 6105 2x18650 charger shown above (aka Cytac or HXY charger) for the first part of my 18650 charging regimen. The reason for this will limited role will become clear with the charging summary graph below.

Although capable of charging at a higher rate, it lack any sort of Constant Current or Constant Voltage charging method, and doesn’t terminate at the end of a charge (i.e. just slowly drops off in charge, even after the light goes green). It does feature two independent channels, though.

CHARGER COMPARISONS

To compare the performance of the chargers, I have directly monitored their charging currents with a data-logging DMM. I have used the measuring method described by HKJ in his excellent Measurement on flashlight overview, for measuring charge current on Li-ion chargers.

For these tests, I have used one of my standard AW protected 18650 (2200mAh) cells. To deplete the cell, I used my regular test bed of a fully-regulated JetBeam Jet-III IBS on Max output – and waited until the cell’s protection circuit was tripped. The cell was then immediately loaded into the charger for testing and recording (i.e. depleted resting voltage typically ~3V).

The graph below is complicated, so let me walk you through each trace.

First off – a note about the early termination on three of the chargers (i.e. ~4.15V on the DSD, WP2, and MP1). I believe this is due to a combination of effects – notably the age of the cell used in testing (one of my “retired” 18650s with a lot of charging cycles on it), and the presence of the DMM in the charging path.

Even though this current-charging measurement setup has a relatively low impedance and a low voltage drop, it still seems to confuse the charger termination when older cells are used. In all three of these cases, when I re-inserted the cell without the DMM, the charging light went red again, and continued until ~4.18-4.19V resting volts. But if I reconnected with the DMM in place, the light stayed green and wouldn’t charge further (strongly suggesting the DMM plays a role in the issue).

Testing further, I noticed these three chargers were variable in their termination points even without the DMM in place. Basically, new cells all terminated ~4.18-4.19V resting volts. But older cells, with a lot cycles on them, terminated anywhere from ~4.13-4.16V resting volts. In each of those cases, re-inserting the cell caused charging to continue (although not necessarily all the way up to 4.19V resting volts, depending on how old the cell was).

Take home message – recycle your old cells once they get a lot of charges on them (i.e >100 cycles).

SUMMARY OF FINDINGS

Pila charger: The Pila curve above is exactly what you want to see. The charger runs initially at a constant-current (~630mA in my case) for ~2.5 hours, and then switches to a constant-voltage mode for the last ~1.5 hour of charging. When the cell is fully charged, the charging light turns green and the current drops to zero. At this point, I pulled out the cell and it read 4.20V resting voltage (i.e. fully charged). Perfect. : thumbsup:

DSD charger: The DSD charger has a fairly constant charging current - ~0.35A at the start of my run, slowly dropped to ~0.30A when the light went green and it terminated the charge 5+ hours later. Note the early termination in my tests (explained above).

DX.6105 charger: The DX.6105 does not use a constant-current method, but simply starts at a high current and gradually drops off (mine started just over 1A, although some have reported lower initial currents, e.g. ~0.75A). Note that the unit does not terminate once the light goes green, but continues to charge at a low rate (140mA and dropping, in my case). This is NOT recommended – 140mA is still a lot of current, and you will quickly over-charge your cells if you do not IMMEDIATELY pull them out of the charger. This is really bad on Li-ions. : 10mins after the light went green, resting voltage was 4.21V on my sample.

[B]Xtar WP2 charger:[B] Although the trace looks a little strange, the WP2 is similar to the Pila in its CC/CV algorithm. For some reason, my DMM kept reporting a drop in current, for a fraction of a second every 2-3 secs or so (which is what gives you that strange looking trace above ). Like the DSD charger, early termination was noted on my admittedly old cell (see explanatory note above)

Note that the WP2 does not actually completely terminate when the light goes green. Although my DMM dropped to zero on my 10A port, when I switched to my DMM’s mA/uA port, I could detect a low current of 39uA. Again, every few seconds or so, it would briefly jump up to a slightly larger current (up to a max of ~300uA). Not sure what this means, but it may be a sign of the monitoring current reported for the WP2. In any case, this low uA current is not likely to be a problem for the cell (i.e. nowhere near as bad as the mA currents of “trickle chargers”).

My main concern here is the relatively high charging rate at the time of termination (i.e. >200mA). That's pretty high, and wouldn't be the best for smaller cells with lower capacity. So while you could fit RCR (16340) in the WP2 with a standard length spacer, this is not the ideal charger for these cells.

WP1 charger: The WP1 starts off at its rated 0.5A charging current. But it doesn’t seem to be a proper CC/CV charger as claimed. Rather, the charging current drops off to around 0.45A reasonably quickly (over ~10 mins) and then seems to slowly decay over time, until it reaches a fully charged state. Like the DSD and WP2, the MP1 showed early termination on my older 18650 cell (see note above).

At the point when the light goes green, the current drops to an ultra-low 5.6uA. This is consistent with what Xtar reports for the MP1 – i.e. drops to a low current (4uA claimed), while it continues to monitor the charge state of the cell. It will supposedly then periodically increases the current, to keep the battery fully topped-up. Up to you if you want such a feature, but this ultra-low uA current shouldn’t be a problem.

Preliminary Discussion:

I will preface my comments by making it clear up-front that I am not an expert on battery chemistry or charging methods. There are many more here with infinitely more experience and knowledge of these matters, and I defer to them for any interpretation of the results.

From my (limited) perspective, the Xtar WP2 seems like a pretty good “poor man’s” Pila, (although it lacks the Pila’s ability to force a manual charge). Overall performance and the charging algorithm seem very similar (although the WP2 terminates at a higher ~200mA current). For the estimated street price of ~$20 (shipping included), you certainly get decent value for your money. Definitely much better performance than a lot of the cheap “trickle”chargers out there!

The MP1 doesn’t live up to its claimed CC/CV charging method, but I suppose the overall performance is reasonable for the price point of ~$10. You also have the option of running off USB, AC or DC power. Note that I never recommend running a sustained charging system off a computer’s USB port – best to stick with AC/DC power. As such, I’m glad to see the adapters included on my sample.

As for termination, I personally can’t stand chargers that don’t properly terminate when the light goes green (e.g my DX.6105). That being said, I’m not overly concerned about the ultra-low uA monitoring charge on both of the Xtar chargers – for all intents and purposes, charging of the cells really stops at this point. But as with all chargers, I recommend you pull the cells out once the light goes green.

I welcome any comments or suggestions on my analysis above.

UPDATE Jan 26, 2010: To allow you better compare the Xtar chargers, below are direct comparisons of the charging current vs charging voltage for each charger, on the same battery (a well-used AW 18650 2200mAh). The blue scale on the left is the charging current, the purple scale on the right is the charging voltage.

As expected, the WP2 shows periodic dips in the charging voltage every 2-3 secs (i.e. just like you see for the charging current). The magnitude of the voltage deflection is a lot less though - I never saw more than ~70mV variance from expected.

Once the light when green and the charger quasi-terminated, the voltage dropped to the resting level of 4.14V.

The voltage trace of the MP1 is about what you would expect. I don't know what algorithm the charger is using exactly, but there were no unexpected major jumps/dips in charging voltage. Thanks to the low (<70mA) charging current when the charger terminated, there's no significant change in the resting/charging voltage (i.e. ends at around 4.15V).

Hope that was helpful!

----

Xtar WP2 and MP1 provided by Xtar for review. All other chargers were personally purchased several years ago.

I won't comment on the MP1, as obviously, this isn't a true CC/CV charger. It looks like the WP2 however, utilizes a pretty good approximation of a CC/CV charging algorithm, ie. as you say, 'a poor man's Pila".

The biggest concern I would have with the WP2, is the relatively high current towards the end of charge, as you noted. This may, or may not really be an issue, however higher current rates towards the end of charge are known to cause more rapid oxidation of the cell's electrodes than lower rates of current. Therefore, I think the Pila is "kinder" to the cells than the WP2, during the final portion of the CV stage.

Two other points that I find questionable, are the current pulses during charge, and the micro current flowing after termination of charge. These oddities may not really be an issue, however they are a departure from the recommended CC/CV algorithm particularly, the current flow after termination.

The current "pulsing" during charge, is in my opinion, obviously not a Constant Current, as is the recommendation for a proper charging algorithm. Also, one of the requirements for a proper charging algorithm is that the charging current drop to zero at charge termination. I agree, that the tiny current level that remains is likely not a big issue, but is yet another departure from the recommend proper charging algorithm.

Another point I'd like to make is that the "fully charged" state of any chemistry of Li-Ion cell, as pertains to charge termination, is determined not by the OC voltage of the cell, but at a predetermined level in the declining charge current rate, during the CV stage. This is why used, or older cells should terminate charge with a lower OC voltage than newer, less used cells, when a proper charging algorithm is used. This prevents unnecessary oxidation of older, or well used cell's electrodes. The actual OC cell voltage of a cell being charged is irrelevant, as pertains to charge termination. This should not be confused with charging circuit voltage, which should be 4.20 +/- 0.05 Volts during the CV stage, for lithium cobalt cells.

Nicely done review Selfbuilt, in particular I love seeing the charging current overlays. One of these days I've got to pick up a logging voltmeter - if you have any ideas on cost effective solutions for that one, available in Canada even better, I'm all ears.

Written by LetThereBeLite on 01-15-2011 09:36 PM GMT

Originally Posted by selfbuilt

The WP2 comes with both standard AC power adapter and DC car adapter. The cabling and overall build seem reasonably good (good length on the cables, reasonably solid, etc.). The battery trays are spring-mounted, and can easily accommodate any size from 14500-18650.

Very nice review Very nice review selfbuilt. Thank you.

I recently purchased a WP2, however mine didn't come with a DC car adapter. I paid $15.99USD (shipping inclusive) and it was shipped from SZ China.

I have no need for a car adapter so it's not a big deal, but now I do wonder if the DC car adapter is a standard--or optional--part of the WP2 package.

Written by Dances with Flashlight on 01-15-2011 11:31 PM GMT

Thanks for all the work Selfbuilt. Much appreciated.

Written by shadowjk on 01-15-2011 11:39 PM GMT

Originally Posted by selfbuilt

Testing further, I noticed these three chargers were variable in their termination points even without the DMM in place. Basically, new cells all terminated ~4.18-4.19V resting volts. But older cells, with a lot cycles on them, terminated anywhere from ~4.13-4.16V resting volts. In each of those cases, re-inserting the cell caused charging to continue (although not necessarily all the way up to 4.19V resting volts, depending on how old the cell was).

This is expected. It's important to note that the chargers are not supposed to terminate at 4.20 resting volts. They're supposed to terminate at 4.2V charging volts, when current drops below their threshold. This is expected. It's important to note that the chargers are not supposed to terminate at 4.20 resting volts. They're supposed to terminate at 4.2V charging volts, when current drops below their threshold.

WP1 charger: The WP1 starts off at its rated 0.5A charging current. But it doesn’t seem to be a proper CC/CV charger as claimed. Rather, the charging current drops off to around 0.45A reasonably quickly (over ~10 mins) and then seems to slowly decay over time, until it reaches a fully charged state. Like the DSD and WP2, the MP1 showed early termination on my older 18650 cell (see note above).

Very good comparison! I got a WP2 about a week ago after hearing from people that its a true CC/CV charger. And it kind of is but its not really a proper ones. Here are a few things I noted and I think you described the exact things above.

Originally Posted by jasonck08

I just got one a couple two days ago, but I'm not too impressed.

I monitored the charge voltage and current with my Fluke DMM.

1) It seems to constantly ramp the current up and down. It charges at 625mA for 1 second then the current will flash to around 100-200mA then back up to 625mA. Maybe it ramps the current up and down to measure voltage.

2) The charge current during the CV stage is too high. A cell @ 4.15v (SOC) is being charged at ~200mA.

3) If the current does drop down to 65mA as old4570 stated, it doesn't do this for more than 30 seconds because I was never able to observe current this low before the cutoff.

On a positive note, it does seem to terminate, and the current going to the cell after the lights are green is so low, it wouldn't hurt the cells to leave them in there for a few extra hours. Mine terminated both cells at 4.17 which is also quite good.

While its better than most chargers on the market, I'll stick with my hobby charger.

The recommended CV cutoff stage for most 18650 cells for example is ~40-50mA. The 200mA of the WP2 is far to high, but is better than some crappy chargers out there. The recommended CV cutoff stage for most 18650 cells for example is ~40-50mA. The 200mA of the WP2 is far to high, but is better than some crappy chargers out there.

Written by selfbuilt on 01-16-2011 11:32 AM GMT

Excellent feedback everyone, appreciate all the insight and comments.

Originally Posted by SilverFox

I prefer a Li-Ion charger to completely shut off after termination, however the effect of micro "trickle" charging has not been studied, so I don't know how much of an issue that is.

Originally Posted by 45/70

Two other points that I find questionable, are the current pulses during charge, and the micro current flowing after termination of charge. These oddities may not really be an issue, however they are a departure from the recommended CC/CV algorithm particularly, the current flow after termination.

Originally Posted by jasonck08

The recommended CV cutoff stage for most 18650 cells for example is ~40-50mA. The 200mA of the WP2 is far to high, but is better than some crappy chargers out there.

Thanks for your detailed and thoughtful comments guys. Yes, I agree, it definitely is a departure from the recommended CC/CV algorithm, and hard to know if it will be a problem. Same goes for that 200mA cut-off current. Thanks for your detailed and thoughtful comments guys. Yes, I agree, it definitely is a departure from the recommended CC/CV algorithm, and hard to know if it will be a problem. Same goes for that 200mA cut-off current.

Originally Posted by tandem

Nicely done review Selfbuilt, in particular I love seeing the charging current overlays. One of these days I've got to pick up a logging voltmeter - if you have any ideas on cost effective solutions for that one, available in Canada even better, I'm all ears.

I used an old Uni-T UT60A for the datalogging above (given to me as a gift over 10 years ago, so I know it was available in Canada). But it uses the old serial RS232 interface (and none of the USB-RS232 adapters I've tried work with the software). More commonly now, I also use a V&A VA18B, which is a USB-based datalogging multimeter. It can be had online fairly cheaply. I used an old Uni-T UT60A for the datalogging above (given to me as a gift over 10 years ago, so I know it was available in Canada). But it uses the old serial RS232 interface (and none of the USB-RS232 adapters I've tried work with the software). More commonly now, I also use a V&A VA18B, which is a USB-based datalogging multimeter. It can be had online fairly cheaply.

I then plot everything in Excel, using the template I developed for my runtime graphs. The sampling rate above was a reading every 30 secs (which I know from the runtimes is good enough for excellent resolution over typical time frames).

Originally Posted by LetThereBeLite

I recently purchased a WP2, however mine didn't come with a DC car adapter. I paid $15.99USD (shipping inclusive) and it was shipped from SZ China.

Hmm, I imagine there will be some variability in what's included, depending on the seller. A pity these don't seem to be available in a consistent fashion at a North American dealer. I full expect the MP1 USB adapters to be variable (they aren't included in the cardboard packaging, but came as extras in baggies). Hmm, I imagine there will be some variability in what's included, depending on the seller. A pity these don't seem to be available in a consistent fashion at a North American dealer. I full expect the MP1 USB adapters to be variable (they aren't included in the cardboard packaging, but came as extras in baggies).

A good point about the USB cables - another reason why this isn't the best way to go. A good point about the USB cables - another reason why this isn't the best way to go.

On the plus side, at least the MP1 terminates at a reasonably low current (i.e. better than the DSD, which is still 300mA).

Written by jasonck08 on 01-16-2011 03:38 PM GMT

Even though the MP1's curve looks nothing like your typical proper CC/CV datasheet curve, it looks like it would do less harm to your cells than the WP2 because of the lower termination current.

Written by tandem on 01-16-2011 03:45 PM GMT

Originally Posted by selfbuilt

The sampling rate above was a reading every 30 secs (which I know from the runtimes is good enough for excellent resolution over typical time frames).

Thanks for answering what was to be my next question. I wondered what the sampling rate was for your measurements. Thanks for answering what was to be my next question. I wondered what the sampling rate was for your measurements.

When I look at a nice clean demonstration of the Pila's charge current I still find it difficult to feel comfortable with the WP2's departure from anything resembling constant in the first phase of the charging program.

Given the sampling rate is twice a minute I wonder what a higher resolution rate would divulge (perhaps nothing different) about the WP2. Along the same lines, I don't suppose you or anyone else has a scope on hand to take a closer look at what is happening there?

I recently purchased a WP2, however mine didn't come with a DC car adapter. I paid $15.99USD (shipping inclusive) and it was shipped from SZ China.

I have no need for a car adapter so it's not a big deal, but now I do wonder if the DC car adapter is a standard--or optional--part of the WP2 package.

Written by selfbuilt on 01-17-2011 07:11 AM GMT

Originally Posted by tandem

Given the sampling rate is twice a minute I wonder what a higher resolution rate would divulge (perhaps nothing different) about the WP2. Along the same lines, I don't suppose you or anyone else has a scope on hand to take a closer look at what is happening there?

Don't have a scope, sorry. As to the sampling rate, the max my datalogger will do is once every 0.5 sec (for the V&A) or once a second (for the Uni-T). But it doesn't matter - the trace would look about the same. Basically, every ~2-3 secs you would see a dip to a lower current. That averages out over the longer timescale as well (i.e. every second to third reading at 30sec intervals is low, on average). Don't have a scope, sorry. As to the sampling rate, the max my datalogger will do is once every 0.5 sec (for the V&A) or once a second (for the Uni-T). But it doesn't matter - the trace would look about the same. Basically, every ~2-3 secs you would see a dip to a lower current. That averages out over the longer timescale as well (i.e. every second to third reading at 30sec intervals is low, on average).

Written by fvdk on 01-18-2011 07:56 AM GMT

Very nice review Selfbuild, thanks !

I will test my WP2 again as I remember that the cut-off current was indeed 60mA when I charged an empty cell.

I did however see a cut-off current close to 200mA when I charged a cell the that was only discharged a little bit.

Would be nice if Miles can explain why the charge current jumps up and down every few seconds. There will probably be a good explanation for it and I think that it can still be considered a (puls modulated) CC but it would be nice to know.

Frans

Written by jasonck08 on 01-18-2011 03:02 PM GMT

My guess about the pulse current from the WP2 is maybe the charger is trying to measure the batteries resting voltage. Either way its kind of a strange thing to do. I just wish Xtar would have spent another 50 cents or a dollar and picked a decent charge IC that is more in line with the proper CC/CV charge method. I built a USB Li-ion charger for $3 that does a better job than these Xtar's!

Written by LetThereBeLite on 01-19-2011 03:30 AM GMT

Originally Posted by jasonck08

My guess about the pulse current from the WP2 is maybe the charger is trying to measure the batteries resting voltage. Either way its kind of a strange thing to do. I just wish Xtar would have spent another 50 cents or a dollar and picked a decent charge IC that is more in line with the proper CC/CV charge method. I built a USB Li-ion charger for $3 that does a better job than these Xtar's!

My own observations made with a DMM on the WP2 charger is that it's also pulse charging. It is not charging at a constant rate. It pulses both in the CC My own observations made with a DMM on the WP2 charger is that it's also pulse charging. It is not charging at a constant rate. It pulses both in the CC and CV stage.

jackson, if that is true--and I have no reason to believe it is not--regarding a $3 li-ion charger better than the WP2, then why don't you actually mass produce them and sell them. It'll be Made in the USA and shipped from the USA. You should make a handsome killing.

Written by jasonck08 on 01-19-2011 04:24 AM GMT

Originally Posted by LetThereBeLite

My own observations made with a DMM on the WP2 charger is that it's also pulse charging. It is not charging at a constant rate. It pulses both in the CC and CV stage.

jackson, if that is true--and I have no reason to believe it is not--regarding a $3 li-ion charger better than the WP2, then why don't you actually mass produce them and sell them. It'll be Made in the USA and shipped from the USA. You should make a handsome killing.

Well its only 400mA current, and only has a single charging channel. And that $3 didn't include any casing or anything, thats just the bare PCB, USB plug and a couple of wires with magnets on them. Well its only 400mA current, and only has a single charging channel. And that $3 didn't include any casing or anything, thats just the bare PCB, USB plug and a couple of wires with magnets on them.

And you are correct about the pulsing during the CC and CV stage. When I put my Fluke DMM on the mode where it averages current, it read around 190-200mA during the CV stage. Still too high. Pulses were as high as 250mA as low as like 20mA. I don't know why its doing this, but my initial guess was it has something to do with reading the cells voltage as I've seen some other chargers ramp up and down... only not NEARLY as frequently as this one!

Written by selfbuilt on 01-19-2011 07:09 AM GMT

Originally Posted by jasonck08

Even though the MP1's curve looks nothing like your typical proper CC/CV datasheet curve, it looks like it would do less harm to your cells than the WP2 because of the lower termination current.

A good point - it's actually preferable over the DSD charger as well (which terminates at 300mA in my testing). Should be able to fit smaller cells in the MP1, with the use of spacers. Might make an acceptable 14500/RCR charger that way ... A good point - it's actually preferable over the DSD charger as well (which terminates at 300mA in my testing). Should be able to fit smaller cells in the MP1, with the use of spacers. Might make an acceptable 14500/RCR charger that way ...

Originally Posted by jasonck08

And you are correct about the pulsing during the CC and CV stage. When I put my Fluke DMM on the mode where it averages current, it read around 190-200mA during the CV stage. Still too high. Pulses were as high as 250mA as low as like 20mA. I don't know why its doing this, but my initial guess was it has something to do with reading the cells voltage as I've seen some other chargers ramp up and down... only not NEARLY as frequently as this one!

Yes, I observed the same thing - it pulses just as much (and just as quickly) in the CV stage as the CC stage (as you can see on my graph). Yes, I observed the same thing - it pulses just as much (and just as quickly) in the CV stage as the CC stage (as you can see on my graph).

Originally Posted by fvdk

I will test my WP2 again as I remember that the cut-off current was indeed 60mA when I charged an empty cell.

I did however see a cut-off current close to 200mA when I charged a cell the that was only discharged a little bit.

Interesting. I tested two cells from completely drained - one "retired" cell with a lot cycles on it, and one cell that has seen reasonable use - and both terminated around 200mA. I haven't tested a relatively new cell, though. Interesting. I tested two cells from completely drained - one "retired" cell with a lot cycles on it, and one cell that has seen reasonable use - and both terminated around 200mA. I haven't tested a relatively new cell, though.

Written by tandem on 01-19-2011 08:44 AM GMT

Originally Posted by jasonck08

Even though the MP1's curve looks nothing like your typical proper CC/CV datasheet curve, it looks like it would do less harm to your cells than the WP2 because of the lower termination current.

True, assuming that a voltage trace for the MP1 (and every other charger) doesn't speak to any faux pas or cell atrocities. It would be informative to see a second chart plotting voltage over time for each of these chargers overlaid as the current over time chart has been done. True, assuming that a voltage trace for the MP1 (and every other charger) doesn't speak to any faux pas or cell atrocities. It would be informative to see a second chart plotting voltage over time for each of these chargers overlaid as the current over time chart has been done.

Why is a constant charge preferred over pulse charging? From other reading I've done, pulse charging has advantages and is the "new" preferred protocol for lithium-ion.

Written by tandem on 01-19-2011 10:58 AM GMT

Cell makers specify CC/CV as the proper charging protocol (eg Panasonic, Sanyo (now owned by Panasonic pending final approval this spring), Samsung, etc). Until they change these directives, CC/CV will and should dominate.

Here's one paper discussing pulse charging of li-ion cells but it doesn't speak only of pulsed charge but pulsed discharge periods during the charging process[1]. Is this an accepted practice by industry?

The conventional lithium-ion battery charging occurs in two steps, the battery is charged at a constant current (e.g. 1/3 C) until the potential reaches the upper voltage limit (4.1 or 4.2 V) followed by constant voltage charging until the current reaches a predetermined small value. The constant voltage charging seriously extends the charging time. It is well-known that lithium ion diffusion in the electrode is the rate-determining step in the charging process. The slow lithium ion diffusion inevitably results in concentration polarization, especially at the high current charging, bringing the battery voltage rapidly to the upper voltage limit.

Alternatively, the constant voltage charging drops the current to the pre-set limit before the active material in the electrode is completely utilized. In order to overcome these problems, the pulse-charging can be used for lithium-ion batteries, where short relaxation periods and short discharge pulses are applied during the charging process. The short relaxation periods and discharge pulses interspersed during the charging process can effectively eliminate the concentration polarization and increase the power transfer rate, thus improving the active material utilization and accelerating the charging process.

The paper concludes that pulse charging combined with pulse discharges during the charging process appears to be beneficial. The paper concludes that pulse charging combined with pulse discharges during the charging process appears to be beneficial.

Without impugning the paper's authors in the slightest I note that the paper was sponsored by a charger manufacturer (which appears to have disappeared since 2001) and I can only assume they produce a product which utilizes a pulse charge-pulse discharge methodology, no doubt designed for specific commercial applications where very high rates of charge are required.

Edit: There do seem to be some IC support out there for pulse charging. There seems to be the potential to make a cheaper charger based on them for a variety of reasons including reduced component count and lower tolerance components required due to reduced heat stress.

Written by fvdk on 01-19-2011 12:32 PM GMT

Originally Posted by tandem

Here's one paper discussing pulse charging of li-ion cells but it doesn't speak only of pulsed charge but pulsed discharge periods during the charging process[1]. Is this an accepted practice by industry?

Is there any evidence the WP2 does that?

Well, I wouldn't call it evidence without further investigation but while monitoring the charge current, every +/- 20 seconds I see it change briefly from (for example) +500 mA to -500 mA. Well, I wouldn't call it evidence without further investigation but while monitoring the charge current, every +/- 20 seconds I see it change briefly from (for example) +500 mA to -500 mA.

I'm running a test charge at the moment to see at which cut-off current the charger terminates but unfortunately, I still have not received the software for the multimeter so I can't post a chart.

Frans

Written by PapaLumen on 01-19-2011 12:49 PM GMT

Very good review.

It would be nice to see a hobby charger included in one of these tests. I use one and i know lots of others do too. Just interested how they will turn out.

Written by selfbuilt on 01-19-2011 01:05 PM GMT

Originally Posted by tandem

True, assuming that a voltage trace for the MP1 (and every other charger) doesn't speak to any faux pas or cell atrocities. It would be informative to see a second chart plotting voltage over time for each of these chargers overlaid as the current over time chart has been done.

Yes, I've had much the same thought. Unfortunately, that doubles the time to do the analysis (since I have to repeat the runs with the electrodes re-placed for voltage monitoring). I'm a little swamped with runtime testing for upcoming light reviews, but I'll see if I can get back to this before too long (at least for the new light Xtar lights). Yes, I've had much the same thought. Unfortunately, that doubles the time to do the analysis (since I have to repeat the runs with the electrodes re-placed for voltage monitoring). I'm a little swamped with runtime testing for upcoming light reviews, but I'll see if I can get back to this before too long (at least for the new light Xtar lights).

Written by 45/70 on 01-19-2011 01:38 PM GMT

Originally Posted by tandem

The paper concludes that pulse charging combined with pulse discharges during the charging process appears to be beneficial.

The pulse charging method has been discussed before. It is nothing all that new. The benefits as I understand it, are twofold. First, Li-Ion cells can be charged faster with the PWM method. Secondly, it is cheaper to implement than a constant current approach. The pulse charging method has been discussed before. It is nothing all that new. The benefits as I understand it, are twofold. First, Li-Ion cells can be charged faster with the PWM method. Secondly, it is cheaper to implement than a constant current approach.

The drawback to pulse charging Li-Ion cells, and likely why the manufacturers of Li-Ion cells don't support it, is that higher current levels towards the end of charge of Li-Ion cells, is detrimental to the cell when applied during the CV stage, at voltages from 4.05 Volt and up. High current rates, even if pulsed, contribute to more rapid oxidation of the cell's electrodes, than do lower current rates during the CV stage. In the interest of charging cells faster, this may be a viable trade off in some applications, such as EVs.

I of course don't know for sure, but I imagine XTAR uses pulse charging (as do many other charger manufacturers), simply because it is cheaper to implement. Similar to drivers for lights, PWM is less expensive than current regulation. This is why there are many more examples of PWM regulated lights, than those using current regulation. Current regulated lights cost a bit more, but are more efficient. The only drawback, aside from the additional cost being, that there is color shift of the LED when driven at lower drive levels, when utilizing current regulation vs. PWM.

Dave

Written by HKJ on 01-19-2011 02:53 PM GMT

Originally Posted by 45/70

I of course don't know for sure, but I imagine XTAR uses pulse charging (as do many other charger manufacturers), simply because it is cheaper to implement. Similar to drivers for lights, PWM is less expensive than current regulation. This is why there are many more examples of PWM regulated lights, than those using current regulation. Current regulated lights cost a bit more, but are more efficient. The only drawback, aside from the additional cost being, that there is color shift of the LED when driven at lower drive levels, when utilizing current regulation vs. PWM.

I do not really believe you price argument, a LiIon charge chip that follows the correct charge curve is only about 50 cents and only need for a few cent in support components. Puls charging would be more expensive to implement. I do not really believe you price argument, a LiIon charge chip that follows the correct charge curve is only about 50 cents and only need for a few cent in support components. Puls charging would be more expensive to implement.

Written by fvdk on 01-19-2011 03:01 PM GMT

Originally Posted by selfbuilt

Interesting. I tested two cells from completely drained - one "retired" cell with a lot cycles on it, and one cell that has seen reasonable use - and both terminated around 200mA. I haven't tested a relatively new cell, though.

Looks like I might have been wrong. Looks like I might have been wrong.

On the test charge that I just did, the cut-off current was 156 mA.

So a bit lower than the 200 mA as observed by you but still too high.

Written by LetThereBeLite on 01-19-2011 03:02 PM GMT

Originally Posted by fvdk

Looks like I might have been wrong.

On the test charge that I just did, the cut-off current was 156 mA.

So a bit lower than the 200 mA as observed by you but still too high.

This is way too high. Don't the specs say 600mah charge (to start) and terminate at 60mah? This is way too high. Don't the specs say 600mah charge (to start) and terminate at 60mah?

Written by 45/70 on 01-19-2011 05:38 PM GMT

Originally Posted by HKJ

I do not really believe you price argument, a LiIon charge chip that follows the correct charge curve is only about 50 cents and only need for a few cent in support components. Puls charging would be more expensive to implement.

That may very well be, HKJ. I'm sure you're way ahead of me when it comes to building electronics. That's just the way I looked at it. That may very well be, HKJ. I'm sure you're way ahead of me when it comes to building electronics. That's just the way I looked at it.

At any rate, I don't think XTAR's charger is the same as tandem's reference described. As far as I know, and I am by no means an expert, these chargers are for specific applications unrelated to flashlight/torch use.

With some tweaking they may work. For example, if the charge is limited to 4.00 Volts, such as they are doing with these pulse chargers for EV's, it may work quite well. Otherwise, until the cell manufactures jump on the bandwagon, I'll pass.

Also, as I said, I doubt that XTAR's versions are really "pulse chargers", as was referenced. If they were, XTAR would surely be telling us all about it. Instead, they simply refer to them as CC/CV chargers.

Dave

Written by selfbuilt on 01-26-2011 02:51 PM GMT

To allow you better compare the Xtar chargers, below are direct comparisons of the charging current vs charging voltage for each charger, on the same battery (a well-used AW 18650 2200mAh). The blue scale on the left is the charging current, the purple scale on the right is the charging voltage.

As expected, the WP2 shows periodic dips in the charging voltage every 2-3 secs (i.e. just like you see for the charging current). The magnitude of the voltage deflection is a lot less though - I never saw more than ~70mV variance from expected.

Once the light when green and the charger quasi-terminated, the voltage dropped to the resting level of 4.14V.

The voltage trace of the MP1 is about what you would expect. I don't know what algorithm the charger is using exactly, but there were no unexpected major jumps/dips in charging voltage. Thanks to the low (<70mA) charging current when the charger terminated, there's no significant change in the resting/charging voltage (i.e. ends at around 4.15V).

Thanks for posting the additional tests. The MP1 although doesn't have the typical CC/CV curve, will be more gentle on cells. I still don't know why they don't pick a proper IC that can be had for less than a buck.

Written by cypherpunks on 01-26-2011 05:02 PM GMT

I don't know what people are getting excited about. True pulse charging is a low duty cycle with a high peak current. You're still averaging 0.6A, but you're delivering it in bursts of 6A followed by 9 pulse times of 0A rest. (I can explain the supposed benefits in lead-acid batteries, but I don't know if Li-ions have anything equivalent to lead sulfate crystal formation.)

What this charger appears to be doing is pausing the charging briefly to measure the cell voltage independent of contact resistance. When you're trying to measure millivolts accurately, it doesn't take a lot of wire and contact resistance for 600 mA of current to induce significant voltage drop. (I just measured the resistance of touching two multimeter probes together as about 0.02 ohm, using a 6.5-digit Agilent DMM in 4-wire mode. That would be 12 mV at 600 mA current.)

When manufacturers talk about a "constant-current/constant-voltage" charge cycle, it really means current-limited, voltage-limited. Charging at exactly the maximum limit 100% of the time is just a matter of efficiency. Charging at a lower current for part or all of the time just increases the charging time correspondingly.

Why people seem to think that the WP2 charging current graph is a problem because "OMG, it's not constant" is beyond me. It's constantly within the limit, and almost always right at it. Nothing in the WP2 charge current graph indicated any problem. The ideal thing would be to have separate force and sense battery contacts and measure the battery terminal voltage exactly, but that's both expensive and fragile. Just stopping charging for a moment to get a more stable measurement seems like a simple enough thing. Maha's well-regarded NiMH charger does the exact same thing.

Written by tandem on 01-26-2011 06:58 PM GMT

I don't think there has been much doubt that the charger is pausing, rather than pulsing, despite the pulse charging tangent the thread went on for a moment. Whether or not the behavior of the WP2 is cause for concern is probably a question for the chemists in the crowd than the EE folks. If we are simply guessing here, my guess it won't matter. But is guessing prudent with a potentially volatile cell chemistry?

You and others who see little cause for concern over the XTAR's charging current pauses are probably correct, but my fertile imagination looks at those pauses and wonders why XTAR decided to go that route. You gave a potential reason. I believe we've all assumed that given the type of product it is (a budget charger) it seems natural to conclude that they chose this implementation for cost saving reasons. Perhaps for ease of voltage sense; or maybe they simply choose a chip with a poor implementation or got a deal on same. Cost savings isn't always a dirty word, but in conjunction with the topic at hand, charging li-ion cells properly, cost savings has earned a bad rep around here and often for good reason.

I suppose if we were to slap a pair of logging meters on to a dozen hobby chargers and find that most of them failed to live up to scrutiny in much the same way cradle style chargers tend to disappoint, *maybe* some of us could feel better about cutting the XTAR some slack.

Maybe... but the li-poly guys have even more fires than we do.

Me, I think I'll stick with known good implementations, at least when it comes to lithium ion and lithium poly cells. To me it simply seems more prudent to me to spend a little more on a unit which meets to a "T" the charging specifications laid out by the ones who really know what's what -- the chemists at the cell manufacturers. Perhaps the difference in this case won't matter but the testing will be on the backs of users as is often the case with products like these.

Written by 45/70 on 01-27-2011 11:32 AM GMT

Originally Posted by cypherpunks

When manufacturers talk about a "constant-current/constant-voltage" charge cycle, it really means current-limited, voltage-limited. Charging at exactly the maximum limit 100% of the time is just a matter of efficiency. Charging at a lower current for part or all of the time just increases the charging time correspondingly.

Why people seem to think that the WP2 charging current graph is a problem because "OMG, it's not constant" is beyond me......

......The ideal thing would be to have separate force and sense battery contacts and measure the battery terminal voltage exactly, but that's both expensive and fragile. Just stopping charging for a moment to get a more stable measurement seems like a simple enough thing. Maha's well-regarded NiMH charger does the exact same thing.

My question, is why does it stop to check voltage? With a proper CC/CV algorithm, this is totally unnecessary. It seems to be a byproduct of a cost savings measure, as tandem alluded to, and it's possible implementation, is as a safeguard to prevent over voltage. This shouldn't be necessary, if a proper algorithm is used. My question, is why does it stop to check voltage? With a proper CC/CV algorithm, this is totally unnecessary. It seems to be a byproduct of a cost savings measure, as tandem alluded to, and it's possible implementation, is as a safeguard to prevent over voltage. This shouldn't be necessary, if a proper algorithm is used.

Li-Ion chargers cannot be compared to NiMH chargers, or vice versa. The charging algorithms are quite different, as are their goals and do not really have all that much in common.

Yes there are voltage drops, not only because of contact resistance but also due to the protection circuit. The only way to

accurately measure the voltage would be with separate contacts on the battery terminals. Feasible with the unprotected batteries but not with the protected one. By removing the current, the battery voltage can be measured. Unfortunately this is not the voltage the battery is subjected to while the current is applied.

Written by nik33134 on 01-29-2011 11:27 AM GMT

Originally Posted by 45/70

My question, is why does it stop to check voltage? With a proper CC/CV algorithm, this is totally unnecessary. It seems to be a byproduct of a cost savings measure, as tandem alluded to, and it's possible implementation, is as a safeguard to prevent over voltage. This shouldn't be necessary, if a proper algorithm is used.

Would a proper algorithm guarantee proper termination 100% even with a bad battery? And if not, what would the risk be? Would a proper algorithm guarantee proper termination 100% even with a bad battery? And if not, what would the risk be?

Written by 45/70 on 01-29-2011 01:05 PM GMT

Originally Posted by BoarHunter

The only way to

accurately measure the voltage would be with separate contacts on the battery terminals. Feasible with the unprotected batteries but not with the protected one. By removing the current, the battery voltage can be measured. Unfortunately this is not the voltage the battery is subjected to while the current is applied.

My question is, why is it necessary for a Li-Ion charger to measure the voltage of a cell My question is, why is it necessary for a Li-Ion charger to measure the voltage of a cell during charge, at all? I know for a fact, that it is not.

When a proper CC/CV charging algorithm is used, the voltage of the cell itself, is totally irrelevant. The only voltage that is important, is the voltage of the charging circuit during the CV stage of the charge. This is specified, as 4.20 Volts +/- 0.05 volt, for a LiCo cell. There is absolutely no reason to check the OC voltage (or an approximation of) the cell during either stage of the charging process, when a proper algorithm is used.

The only reason it may be necessary for a charger to check the cell's OC voltage during charging, is if it pulse charges the cell with a voltage potentially higher, than the maximum allowed CV voltage parameter (4.25 volts), eg. 4.60 Volts, so as to avoid a possible over voltage situation. If this is the case with the XTAR, and I suspect it is, this means the charger does not use a proper CC/CV algorithm, but a "synthesized" one using PWM, with an added safeguard to prevent possible overcharging of the cell.

I say this all the time, but it never seems to stick with a lot of folks, but the termination of charge of a Li-Ion cell when a proper CC/CV algorithm is used, has nothing to do with the OC voltage of the cell. Charge is terminated at a predetermined point in the declining current value during the CV stage, not voltage.

In addition to the declining current value during the CV stage being necessary, so as to avoid over oxidization of the electrodes within the cell, the CV voltage (charging circuit voltage, not the cell's OC voltage) limits the maximum possible OC voltage of the fully charged cell, to 4.20 volts +/- 0.05 Volts. These two points are major advantages to the use of a proper charging algorithm.

And again, a cell's OC voltage has absolutely nothing to do with charging Li-Ion cells, when a proper charging algorithm is used. So, if voltage checking is actually deemed necessary to implement, the charger is likely not using a proper algorithm, but rather an adapted, "sorta" one. I'm not saying it won't work, just that it deviates from the Li-Ion cell manufacturer's guidelines for charging Li-Ion cells, and opens up the possibility of safety issues, IMO, that do not exist, if a properly implemented charging algorithm is used.

My intent in this, and other threads is not to "bash" XTAR in any way. I merely point out what seem to me, to be issues that differ from the recommended guidelines for charging Li-Ion cells. They (XTAR) seem to be on the right track, more so than most other companies offering Li-Ion chargers anyway.

Dave

Written by 45/70 on 01-29-2011 01:44 PM GMT

Originally Posted by nik33134

Would a proper algorithm guarantee proper termination 100% even with a bad battery? And if not, what would the risk be?

Hi nik. I'm not sure I can answer your question without knowing what you mean by a "bad cell". You really shouldn't be attempting to charge a "bad cell" in the first place. Hi nik. I'm not sure I can answer your question without knowing what you mean by a "bad cell". You really shouldn't be attempting to charge a "bad cell" in the first place.

Seriously, before you attempt to charge a Li-Ion cell, you should know something about the condition of the cell. A lot can be determined about a cell's condition by simply measuring the cell's voltage with a reasonably accurate voltage meter, or DMM. If the cell's voltage is less than 2.50 Volts, for example, the cell is likely damaged, and a charge attempt should not be made.

If a cell passes the voltage check, but begins to get excessively warm during the charge process, this is another sign that the cell is damaged, or "worn" due to increased internal resistance, and the charge should be aborted. Any time a Li-Ion cell is being charged, it is pertinent to observe the charging process closely and watch for anomalies, such as excessive heating of the cell, or hissing due to venting gasses etc. during charge. This is particularly true when attempting to charge cells of unknown condition.

In any event, a "bad cell" will most likely begin to heat up during charging. This is the most common warning sign that something isn't right, and the charge should be terminated. If a cell that is showing warning signs is left on the charger, and not removed, the cell could go into "thermal runaway" and "vent with flame" This applies not only to LiCo Li-Ion cells, but all the other chemistries of Li-Ion cells as well. Although LiMn and LiFe Li-Ion cells are likely to be somewhat less "exciting", these too, could vent if a bad cell were to over heat while charging. This is why it is always recommended to never leave Li-Ion cells charging while unattended. The chances of an incident are relatively low, but why take a chance?

A charger that utilizes a proper charging algorithm is your best bet when charging Li-Ion cells. The safeguards built into a decent Li-Ion charger utilizing a proper charging algorithm go a long way towards the safe use of Li-Ion cells. It still pays to be an astute observer however, and keep a close eye on the charging process, regardless of the charger being used.

As cypherpunks pointed out, a CC/CV charging cycle is really a current-limited, voltage-limited charging cycle. This means the charger output should never exceed either of these limits during the charging cycle. It should be noted that these are charger limits that basically define the CC/CV charging cycle. Basically a CC/CV cycle will charge at the current limit until it reaches the voltage limit, after which the charger is held at the voltage limit (this is a voltage limit of the charger itself, NOT the battery voltage!).

In addition to the CC/CV charging, Li-ion cells recommend termination of the charging cycle once the current drops to a certian point.

Now IF a charger is using a CC/CV charging cycle, but stops the charging cycle BEFORE it drops to the recommended point, then the charger will be terminating a bit early (undercharging). Personally I would be much more concerned if the charger was overcharging.

If the early termination on the WP2 is caused by the DMM, then without the DMM the charge current may continue to drop beyond what is shown in the graphs. But, IF the early termination is caused by the DMM, then it would SEEM to suggest that some battery measurements are being used by the charger to help determine the termination point. This may not be a bad thing as long as the charger has a current limit, voltage limit, and doesn't cause the charger to overcharge.

If the early termination on the WP2 is caused by the DMM, then without the DMM the charge current may continue to drop beyond what is shown in the graphs. But, IF the early termination is caused by the DMM, then it would SEEM to suggest that some battery measurements are being used by the charger to help determine the termination point. This may not be a bad thing as long as the charger has a current limit, voltage limit, and doesn't cause the charger to overcharge.

The early termination is not due to the DMM, I get the same result in my review of it, where I only introduce a 0.01 ohm resistance in series with the battery.

XTAR is going to release the New Updated WP2, I still like to offer this new product's reviewing opportunity to HKJ and Selfbuilt once it is announced released since your guys job is excellent splendid, can I have the honor?

Selfbuilt, have you had a chance to take a look at the 4Greer offerings at Kit-tronics in Quebec? Being your next door province and Greer being a nice guy perhaps he'll send you some units to test. I use mine and it works very well, but I'd like to know exactly how it handles the charging process.